The present invention is directed to recording sheets suitable for use in printing processes. More specifically, the present invention is directed to recording sheets which have been coated with a solution containing a surfactant in the lamellar phase, said recording sheets being particularly suitable for printing with aqueous ink compositions. One embodiment of the present invention is directed to a recording sheet which comprises a substrate and a coating thereon comprising water and a surfactant capable of exhibiting a liquid crystalline phase in water at a temperature of about 25.degree. C. or higher, said coating containing the water and surfactant in relative concentrations such that upon addition of water to the coating, the surfactant undergoes a phase change, thereby increasing the viscosity of the coating. Another embodiment of the present invention is directed to a recording sheet which comprises a substrate and a surfactant which is C.sub.x H.sub.(2x+1) (OCH.sub.2 CH.sub.2).sub.y A, ##STR1## wherein x is an integer of from about 8 to about 22, y is an integer of from 0 to about 14, each R is, independently of the others, hydrogen or an alkyl group, and A is a terminal functional group. Yet another embodiment of the present invention is directed to a recording sheet which comprises a substrate and a coating thereon comprising a surfactant, said surfactant being in a lamellar liquid crystalline phase.
Ink jet printing systems generally are of two types continuous stream and drop-on-demand. In continuous stream ink jet systems, ink is emitted in a continuous stream under pressure through at least one orifice or nozzle. The stream is perturbed, causing it to break up into droplets at a fixed distance from the orifice. At the break-up point, the droplets are charged in accordance with digital data signals and passed through an electrostatic field which adjusts the trajectory of each droplet in order to direct it to a gutter for recirculation or a specific location on a recording medium. In drop-on-demand systems, a droplet is expelled from an orifice directly to a position on a recording medium in accordance with digital data signals. A droplet is not formed or expelled unless it is to be placed on the recording medium.
Since drop-on-demand systems require no ink recovery, charging, or deflection, the system is much simpler than the continuous stream type. There are two types of drop-on-demand ink jet systems. One type of drop-on-demand system has as its major components an ink filled channel or passageway having a nozzle on one end and a piezoelectric transducer near the other end to produce pressure pulses. The relatively large size of the transducer prevents close spacing of the nozzles, and physical limitations of the transducer result in low ink drop velocity. Low drop velocity seriously diminishes tolerances for drop velocity variation and directionality, thus impacting the system's ability to produce high quality copies. Drop-on-demand systems which use piezoelectric devices to expel the droplets also suffer the disadvantage of a slow printing speed.
The other type of drop-on-demand system is known as thermal ink jet, or bubble jet, and produces high velocity droplets and allows very close spacing of nozzles. The major components of this type of drop-on-demand system are an ink filled channel having a nozzle on one end and a heat generating resistor near the nozzle. Printing signals representing digital information originate an electric current pulse in a resistive layer within each ink passageway near the orifice or nozzle, causing the ink in the immediate vicinity to evaporate almost instantaneously and create a bubble. The ink at the orifice is forced out as a propelled droplet as the bubble expands. When the hydrodynamic motion of the ink stops, the process is ready to start all over again. With the introduction of a droplet ejection system based upon thermally generated bubbles, commonly referred to as the "bubble jet" system, the drop-on-demand ink jet printers provide simpler, lower cost devices than their continuous stream counterparts, and yet have substantially the same high speed printing capability.
The operating sequence of the bubble jet system begins with a current pulse through the resistive layer in the ink filled channel, the resistive layer being in close proximity to the orifice or nozzle for that channel. Heat is transferred from the resistor to the ink. The ink becomes superheated far above its normal boiling point, and for water based ink, finally reaches the critical temperature for bubble formation or nucleation of around 280.degree. C. Once nucleated, the bubble or water vapor thermally isolates the ink from the heater and no further heat can be applied to the ink. This bubble expands until all the heat stored in the ink in excess of the normal boiling point diffuses away or is used to convert liquid to vapor, which removes heat due to heat of vaporization. The expansion of the bubble forces a droplet of ink out of the nozzle, and once the excess heat is removed, the bubble collapses on the resistor. At this point, the resistor is no longer being heated because the current pulse has passed and, concurrently with the bubble collapse, the droplet is propelled at a high rate of speed in a direction towards a recording medium. The resistive layer encounters a severe cavitational force by the collapse of the bubble, which tends to erode it. Subsequently, the ink channel refills by capillary action. This entire bubble formation and collapse sequence occurs in about 10 microseconds. The channel can be refired after 100 to 500 microseconds minimum dwell time to enable the channel to be refilled and to enable the dynamic refilling factors to become somewhat dampened. Thermal ink jet processes are well known and are described in, for example, U.S. Pat. No. 4,601,777, U.S. Pat. No. 4,251,824, U.S. Pat. No. 4,410,899, U.S. Pat. No. 4,412,224, and U.S. Pat. No. 4,532,530, the disclosures of each of which are totally incorporated herein by reference.
U.S. Pat. No. 5,492,559, filed concurrently herewith, entitled "Liquid Crystalline Microemulsion Ink Compositions," with the named inventors John F. Oliver, Marcel P. Breton, Stig E. Friberg, Raymond W. Wong, and William M. Schwarz, the disclosure of which is totally incorporated herein by reference, discloses an ink composition which comprises an aqueous phase, an oil phase, an oil-soluble dye, and a surfactant, said ink exhibiting a liquid crystalline gel phase at a first temperature and a liquid microemulsion phase at a second temperature higher than the first temperature.
While known compositions and processes are suitable for their intended uses, a need remains for improved recording sheets suitable for use with aqueous recording inks. In addition, there is a need for recording sheets which, when employed with aqueous inks, exhibit images with sharp line edges and minimal line growth. Further, a need exists for recording sheets which, when employed with aqueous inks, exhibit acceptable dry times. Additionally, there is a need for recording sheets which exhibit reduced intercolor bleed when images of two different colors are printed in close proximity to each other. There is also a need for recording sheets which are particularly suitable for use in thermal ink jet printing processes. In addition, a need remains for recording sheets which, when employed in ink jet printing processes, exhibit little or no cockle or curl. Further, there is a need for transparent recording sheets suitable for ink jet printing processes wherein the ink exhibits little or no beading on the recording sheet and generates high quality images.